Significant Agreements - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended		12 Months Ended
	May 31, 2009	Oct. 30, 2007	Dec. 31, 2013
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration and license agreement date			May 2003
Agreement expiration year			2006
Obligated to make milestone payments			$ 23.4
Isis Pharmaceuticals, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Obligated to make milestone payments			21.1
Licensed patent expiration date			Mar. 27, 2028
Sarepta Therapeutics, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Licensed patent expiration date			Sep. 09, 2014
March 2008 Agreement | Ercole Biotechnology, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of share issued to acquire company, shares			5,811,721
Number of share issued to acquire company, value			8.4
Business acquisition liabilities assumed			1.8
Warrants issued to purchase common stock			0.4
Charley's Fund Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development grant awarded	5.00	2.45
Aggregate amount of research grant			3.4
Revenue recognized from research grant			0.1
Deferred grant payment			$ 3.3